The present invention relates to a PM type of stepping motor and a method of manufacturing a yoke for use in it.
In recent years, there have been greater demands for stepping motors capable of realizing more power savings and higher outputs. In a conventional stepping motor, its stator yoke and its frame yoke are made from pure iron (soft magnetic iron plate, SUY), a cold rolled steel plate (SPC), an electrolytic zinc-coated steel (SEC) or the like. However, it has been found out that any of these materials has a superior direct-current magnetic field characteristic but shows an inferior alternating-current magnetic characteristic during driving of an actual motor. In other words, if such a plate material is subjected to a varying magnetic field, its electrical resistivity becomes low and a large number of eddy currents occur, so that its iron loss becomes remarkably large. As a driving frequency becomes higher, the iron loss becomes more remarkable. This causes a lowering in the efficiency of a stepping motor, and becomes a bottleneck which hinders an improvement in the efficiency which is needed for promoting the battery driving of office automation equipment.
To solve the above conventional problem, various proposals have been made, such as proposals to employ silicon steel or soft ferrite for a stator yoke or a frame yoke (Japanese Utility Mode Laid-Open Nos. 3-104077/1991, 62-135577/1987 and the like) or a proposal to form through-holes in part of a stator yoke (the flow passage of eddy currents) (Japanese Patent Laid-Open No. 3-283049/1991).
However, the above-noted conventional stepping motors respectively have problems which will be described below, and none of the stepping motors can completely solve the problem of efficiency lowering due to eddy currents.
Specifically, the silicon steel is difficult to work by bending (drawing) compared to the aforesaid SUY and SPC, and is further difficult to treat by rust preventive plating. To improve the workability of the silicon steel, the amount of Si to be added may be decreased. However, in this case, the obtained electrical resistivity will lower and an eddy-current decreasing effect will be lost.
The use of soft ferrite can only provide a low saturation magnetic flux density which is not more than one-third of that obtainable from SUY or SPC, so that no sufficient output torque can be obtained. In addition, the soft ferrite cannot be bent and its mechanical strength is week, and, further, it is not suitable for precision machining.
In the arrangement in which through-holes are formed in part of a stator yoke, as the number of through-holes formed increases, the output torque tends to lower, and the number of working steps increases and a cost increase is incurred. In addition, although the through-holes are formed in a portion other than comb-tooth-shaped magnetic poles, no substantial effect can be obtained because of the absence of major variations in magnetic flux or major eddy-current loss in such portions.
The present invention has been made in light of the above-described background, and its object is to provide a stepping motor which is capable of solving the above-described problems and of minimizing eddy currents and providing highly efficient and stable characteristics, and which has good workability, as well as to provide a method of manufacturing a yoke for use in the stepping motor.
To achieve the above object, a stepping motor according to the present invention comprises a rotor made from a permanent magnet magnetized to have multiple magnetic poles, a stator yoke having a plurality of comb-tooth-shaped magnetic poles and opposed coaxially to the rotor, an excitation coil fitted on an external circumference of the comb-tooth-shaped magnetic poles of the stator yoke, and a tube-shaped frame yoke which surrounds the excitation coil and the stator yoke, and at least part of the yoke is formed of an Fe--Cr alloy which essentially consists of Fe and contains 9.0-18.0 wt % Cr and trace additions which are not more than 0.02 wt % C, not more than 0.7 wt % Si, not more than 0.7 wt % Mn, not more than 0.04 wt % P, not more than 0.005 wt % S, not more than 0.5 wt % Ni, not more than 0.02 wt % N, not more than 0.01 wt % 0, not more than 4.0 wt % Al, the Fe--Cr alloy having a ferrite single-phase structure whose F value defined by the following expression is not less than 0 and not more than 8. EQU F value=Cr+Si+2.1Al-37.0(C+N)-2.0Ni-0.6Mn-10.8
(where the unit of each composition is wt %.) PA2 (where the unit of each composition is wt %.) The F value is an index of the structure stability of ferritic stainless steel. If the F value has a minus sign, no stable ferrite structure is obtained and no good magnetic characteristic is obtained. Therefore, the F value needs to be greater than 0. However, as the F value becomes larger, the magnetic flux density of the ferritic stainless steel becomes lower and a motor characteristic is lowered. An experiment has shown that the upper limit of the F value is 8.0. Therefore, the F value is limited to a range between 0 and 8.
In another aspect of the invention, at least part of the yoke is formed of an Fe--Cr alloy which essentially consists of Fe and contains 9.0-18.0 wt % Cr and trace additions which are not more than 0.02 wt % C, not more than 0.7 wt % Si, not more than 0.7 wt % Mn, not more than 0.04 wt % P, not more than 0.005 wt % S, not more than 0.5 wt % Ni, not more than 0.02 wt % N, not more than 0.01 wt % 0, not more than 4.0 wt % Al and 0.01-0.4 wt % Ti, the Fe--Cr alloy having a ferrite single-phase structure whose F value defined by the following expression is not less than 0 and not more than 8. EQU F value=Cr+Si+2.1(Al+Ti)-37.0(C+N)-2.0Ni-0.6Mn-10.8
Cr is an element indispensable for ensuring the anticorrosion characteristic required for the stepping motor. If the Cr content is less than 9.0 wt %, the stepping motor cannot have rust preventive performance which is needed in normal use environments. On the other hand, if a large amount of Cr is contained, the magnetic flux density becomes low and the magnetic characteristic is degraded. If the Cr content exceeds 18.0 wt %, no desired motor performance can be obtained. Therefore, the Cr content is limited to not less than 9.0 wt % and not more than 18.0 wt %.
The reason why the respective contents of the trace additions are limited to the aforesaid ranges is as follows. At present, any of the trace additions is an inevitable impurity which cannot be completely eliminated during manufacture of the Fe--Cr alloy. However, if the trace additions can be completely eliminated, as long as the F value is satisfied, the content of a predetermined substance may be zero, i.e., the predetermined substance may not be added.
From among the inevitable impurities, C, P, S, Ni, N and O are elements that degrades magnetic characteristics, and anticorrosion property. Therefore, it is desirable to eliminate those elements. On the basis of the results of various experiments, the upper limit values of the contents of the undesirable elements were determined as described below for desired characteristics suited to the stepping motor.
C easily forms a carbide to degrade the magnetic characteristic and the anticorrosion characteristic The upper limit of the C content is set to 0.02 wt %, because if the C content exceeds 0.02 wt %, the performance required for the stepping motor which is a final product cannot be obtained P is an element which degrades the magnetic characteristic, and if the P content exceeds 0.04 wt %, the performance required for the stepping motor which is a final product cannot be obtained. For this reason, the upper limit of the P content is set to 0.04 wt %. Since S is an impurity element which easily forms a sulphide and degrades the magnetic characteristic, the S content needs to be suppressed. The upper limit of the S content is set to 0.005 wt %, because if the S content exceeds 0.005 wt %, no desired motor performance can be obtained. Ni is an austenite generating element which degrades the magnetic characteristic, and the limit of the Ni content is set to 0.5 wt %, because if the Ni content exceeds 0.5 wt %, no desired motor performance can be obtained. Since N easily forms a nitride together with Al, Ti or the like and degrades the magnetic characteristic, the N content needs to be suppressed. The upper limit of the N content is set to 0.02 wt %, because if the N content exceeds 0.02 wt %, no desired performance can be obtained. Since 0 is an impurity element which easily forms an oxide and degrades the magnetic characteristic, the 0 content needs to be suppressed. The upper limit of the 0 content is set to 0.01 wt %, because if the 0 content exceeds 0.01 wt %, no desired motor performance can be obtained.
Although Si, Mn, Al and Ti are also contained as trace inevitable impurities, they are positively added to realize the desired characteristics, or to ensure the stability of the composition, or to meet the requirements of manufacturing processes.
Since Mn is an element which is needed for deoxidation to be conducted during steel manufacture, it is preferable to add Mn by a predetermined amount. However, Mn is an element which degrades the magnetic characteristic. If the Mn content exceeds 0.7 wt %, the deoxidation effect is saturated and no predetermined motor performance is obtained. Therefore, the upper limit of the Mn content is set to 0.7 wt %.
Al is an element to be added as a deoxidizing agent, and has the action of decreasing impurities through deoxidation to improve the magnetic characteristic. The upper limit of the Al content is set to 4.0 wt %, because if the Al content exceeds 4.0 wt %, the magnetic characteristic is lowered.
Ti is an element which forms a carbide and a nitride together with C and N and is useful for ensuring the ferrite single-phase structure. Even if an extremely small amount of Ti is added, a remarkable effect can be obtained. Therefore, 0.01 wt % or more Ti improves the stability of the composition (F value) and mass-productivity.
However, if the Ti content exceeds 0.4 wt %, the aforesaid effect is saturated and the merit of the addition of expensive Ti is lost. Therefore, the upper limit of the Ti content is set to 0.4 wt %.
Si is an element which is needed for deoxidation during steel manufacture, similarly to Mn, and is also a ferrite generating element, similarly to Ti. Accordingly, although it is desirable to add a predetermined amount of Si for the same reason as each of the aforesaid elements, the contents of C, N, Mn and Ni in a steel according to the present invention are made smaller than in normal steel, so that an excess amount of Si need not be added and the addition of approximately 0.7 wt % Si is sufficient. If the Si content exceeds 0.7 wt %, its effect is saturated and the merit of the addition of Si is lost. Therefore, the upper limit of the Si content is set to 0.7 wt %.
In a method of manufacturing a yoke according to the present invention, which is used in the stepping motor having the above-described arrangement, a plate material made of the Fe--Cr alloy having the aforesaid composition is worked on a press into a predetermined shape, followed by annealing, preferably between 700.degree. C. and 1,200.degree. C. Specifically, if the annealing temperature is less than 700.degree. C., no substantial strain is removed and an improvement in the magnetic characteristic is not expected. As the magnetic annealing temperature is made higher, more strain is eliminated, and in the present invention, since the quality of the material to be used is controlled on the basis of the F value, the material has a stable ferrite structure. Accordingly, even if the annealing temperature is made high, no martensite is precipitated and the magnetic characteristic is improved (saturated at a constant value) However, if the annealing temperature exceeds 1,200.degree. C., an annealing furnace is remarkably degraded and magnetic annealing itself becomes difficult. For this reason, the upper limit of the annealing temperature is set to 1,200.degree. C. Since the Fe--Cr alloy which has the ferrite single-phase structure having the desired composition ratio is used for the yoke, an electrical resistivity p becomes 5-10 times that of SUY or SPC. Since eddy currents are inversely proportional to the electrical resistivity of the yoke, in the stepping motor according to the present invention, the occurrence of the eddy currents is minimized for example while an alternating-current magnetic field is present during the driving of the motor. Accordingly, its iron loss decreases and its output torque relative to its input power increases, so that a stepping motor of low power consumption can be realized. In addition, since the eddy currents are prevented from easily flowing, a reverse magnetic field decreases, and if the input current of the motor is constant, the total quantity of magnetic flux increases and the output torque of the motor increases. In other words, the same output torque can be obtained from a reduced amount of current which flows through the excitation coil, so that the copper loss of the motor can be decreased. Since the iron loss and the copper loss are decreased in this manner, the heat generation of the motor itself can be suppressed.
In addition, since the Fe--Cr alloy has good workability, conventional manufacturing facilities for Fe alloys can be used to form a yoke of predetermined shape. Since the compositions of individual materials are controlled on the basis of their F values the lower limits of which are set to 0, the ferrite structure of the Fe--Cr alloy is stable, and its workability is extremely good so that the Fe--Cr alloy can also be subjected to drawing.
For this reason, the Fe--Cr alloy can be used for both a stator yoke and a frame yoke. If a predetermined Fe--Cr alloy is used in at least part of either of such yokes, the characteristics of that part can be improved. In particular, it is most effective to use the aforesaid Fe--Cr alloy for the stator yoke which undergoes a large variation in magnetic flux density.
In general, when a yoke is to be manufactured from the Fe--Cr alloy having the aforesaid composition, a plate material is formed into a stator yoke having at least portions bent in the shape of comb-tooth-shaped magnetic poles, by press working, or a plate material is formed into a frame yoke by drawing. The strain of such yoke due to residual stress applied during the press working degrades the magnetic characteristics of the yoke. Specifically, an increase in coercive force (Hc) and a decrease in permeability (.mu.) occur in the yoke. The increase in Hc leads to an increase in hysteresis loss, while the decrease in permeability leads to a decrease in magnetic flux density, so that output torque and efficiency which are motor characteristics are lowered, i.e., the effect of the use of the aforesaid Fe--Cr alloy cannot be fully utilized. Incidentally, the motor characteristics of a stepping motor manufactured by using yokes formed by press working according to the present invention are fully improved compared to a stepping motor using a conventional steel, such as SPC.
However, if magnetic annealing based on the present invention is applied to such a yoke, the strain of the yoke due to residual stress is removed and the recovery of the coercive force (Hc) and the permeability (.mu.) is realized, so that the output torque and the efficiency of the stepping motor are improved.
The extent of this improvement is large compared to the conventional steels SUY and SPC. This is because a primary cause of a decrease in the magnetic flux density of such a conventional steel in an alternating-current magnetic field during motor driving is eddy currents which are not affected by magnetic annealing. It is generally known that hysteresis loss is proportional to the 1.6th power of magnetic flux density, and since the conventional steel whose magnetic flux density is lowered by eddy currents involves hysteresis loss smaller than that of the steel according to the present invention, the effect of magnetic annealing on the conventional steel is small. On the other hand, since the steel according to the present invention involves a far smaller eddy-current loss than the conventional steel, the magnetic flux density of the former steel is higher so that its hysteresis loss due to working becomes larger. In other words, because of the larger hysteresis loss, the effect of magnetic annealing on the steel according to the present invention becomes larger and the necessity of the magnetic annealing becomes higher.